The work on standardization of the Long Term Evolution (LTE) concept is currently ongoing within the third generation partnership program (3 GPP). In LTE the downlink will support channel dependent scheduling in both the time and frequency domains. The radio base station in the LTE network denoted evolved NodeB (eNodeB) will transmit reference signals that the mobile stations also termed User Equipments (UEs) use to determine the downlink channel quality. The UEs will send Channel Quality Indication (CQI) reports back to the eNodeB that are used by a scheduler located in the eNodeB for scheduling resources within the cell that the eNodeB serves.
In the LTE network the UE is specified to be able to transmit different types of CQI reports. For example the UE will have the capability to transmit a full CQI reports and partial CQI reports. Full CQI report covers the whole downlink transmission bandwidth but they may have different frequency resolution, they may be filtered and processed in different ways, and they may be encoded in different ways, see also 3 GPP TS 36.213 V8.2.0, “Physical layer procedures”.
A scheduling function in the eNodeB controls, for each time instant, to which users the shared resources should be assigned. It also determines the data rate to be used for each link, which is also called link adaptation. Both downlink and uplink transmissions are subject to fast scheduling, taking into account the instantaneous traffic pattern and radio propagation characteristics of each UE.
The possibility for channel-dependent scheduling in the frequency domain is particularly useful at low terminal speeds, in other words when the channel characteristic is varying slowly in time. For the downlink, each terminal measures and reports an estimate of the instantaneous channel quality to the eNodeB by sending CQI reports. Based on the channel-quality estimate, the downlink scheduler can assign resources to the users, taking the channel qualities into account. In principle, a scheduled UE can be assigned an arbitrary combination of 180 kHz wide resource blocks in each 1 ms subframe.
Furthermore, CQI reports used together with Single Input Single Output (SISO), Multiple Input Single Output (MISO), Single Input Multiple Output (SIMO), or Multiple Input Multiple Output (MIMO) transmission could also be different for the different transmission cases. To exemplify, for MIMO a CQI report may e.g. include pre-coding weights to be used by the eNodeB multiple antenna transmission scheme.
The LTE uplink is based on single-carrier modulation and uses frequency division multiple access (FDMA) and time division multiple access (TDMA) principles. The LTE uplink consists of physical uplink control channels and data channels that are orthogonally frequency multiplexed. The single-carrier property of the LTE uplink makes it impossible for a UE to transmit on a physical control channel and a physical data channel in the same transmission-time-interval (TTI). Hence if a UE is transmitting data on a physical data channel the CQI that must be sent in the same TTI must also be sent on the physical data channel. When the CQI is transmitted on the physical data channel it is multiplexed with data. When there is no data to transmit on the physical data channel the CQI report is multiplexed with padding bits. The CQI can also be transmitted periodically on the PUCCH and the periodicity is configured through Radio Resource Control (RRC).
The current assumption in 3 GPP regarding the CQI reporting on the physical control channel is that a maximum of approximately 10 bits can be transmitted per UE and TTI This number of bits leaves little space for fine granular CQI frequency domain information and MIMO information. On the physical data channel it will most likely be possible to transmit more bits.
When a UE is scheduled for data transmission on the physical channel it is signaled an uplink grant including resource block allocation and transport format selection. The uplink grant also includes 1 bit which indicates if a CQI report should be included in the uplink transmission or not. The format of this CQI report is Radio Resource Control (RRC) configured.
The transmission of a CQI report is important. The size of the CQI report shall be sufficiently big so that it can represent sufficiently accurate channel quality. It shall be able to transmit with sufficient frequency so that the reported channel quality is not outdated. However, since CQI report consumes resources in the uplink, inefficient CQI reporting will sacrifice uplink transmission capacity and therefore results in reduced capacity in the uplink.
Hence there exist a need for a method and a node that improve the procedure by means of which it is determined when CQI reports are transmitted from the UE.